1. Field of the Invention
The invention relates to a process for producing a casing providing a screen against electromagnetic radiation according to the preamble of claim 1, and a casing according to the preamble of claim 6.
2. Description of the Prior Art
Electronic components and also measuring, detection and similar devices sensitive to interference by electromagnetic radiation require a screen against the electromagnetic fields present at the operational site in order to ensure a problem free operation.
They are therefore accommodated in screening cases which comprise conductive material in the walls and act as a Faraday cage.
Such casings are also used for equipment or components which, themselves, emit electromagnetic rays that must be excluded from the environment, in order for example, to prevent the emission of secret information or the malfunction of external appliances.
Today such a screening against the emission or irradiation of EMI must be more effective the more electronic apparatuses are operated and the greater the proximity at which these apparatuses must operate next to each other. Finally, the continuous increase in performance and sensitivity of such equipment also necessitates an improvement of the screening measures for which increasingly less space is available, since the relevant appliances must, moreover, be miniaturised. Thus, apart from the actual operational properties of electronic appliances, the "electromagnetic compatibility" is today a significant factor determining quality.
If, as is mostly the case in practice, the casings are multi-part constructions which must be capable of being opened occasionally (e.g. in order to renew the energy source or for maintenance purposes), it is necessary to provide the parts of the casing to be separated from each other during opening and to be reconnected again during closing, with elastic conductive seals in order to achieve an effective screening.
Whilst, on the one hand, spring-like metal seals are known for this purpose, they are, however, comparatively expensive to construct and their operability may be greatly affected by oxidation and soiling.
Furthermore, resilient sealing profiles made of elastomer which is conductive or has been made conductive, which has been mixed with carbon or metal particles in order to make it conductive, are known from e.g. U.S. Pat. No. 4,659,869 or DE-OS 28 27 676.
Such sealing profiles are normally manufactured as separate seals. They may be moulded or extruded as a continuous section and then placed into the casing to be screened.
This is a labour-intensive operation and especially in small casings it causes difficulties as seals with correspondingly small dimensions are difficult to handle. The provision of suitable guides (grooves) which facilitate the mounting on the casing requires an unreasonable amount of space and is thus a hindrance to the further miniaturisation of the appliances.
Intricately moulded seals likely to be needed for special casings require specific positioning equipment which makes the manufacture of the casing altogether more expensive. The exact positioning is, moreover, time-consuming and necessitates additional inspection.
The hot-moulding, in moulds, of such screening profiles onto the relevant casing portions or parts and the setting at a relatively high temperature and/or high pressure is also known.
This process cannot be used with parts sensitive to pressure and/or temperature such as printed circuit boards or metallised plastics casings and, as a result of the low tear resistance of the related materials, problems arise during the removal from the mould resulting in a relatively high number of rejects and, more particularly in intricately shaped casings and seals, also frequently necessitating time-consuming and labour-intensive machining on the pressed-out edges.